All animals have evolved in an environment rich in microorganisms, many of which exist in intimate associations that shape the animal hosts'development and normal physiology. We have established a germfree (GF) zebrafish model to study the role of the indigenous microbiota in gut development. We find that the GF zebrafish digestive tract exhibits multiple developmental defects, including failure to express mature patterns of alkaline phosphatase (IAP) activity and Gala1,3Gal containing glycans. We have demonstrated that purified lipopolysaccharide (LPS) is sufficient to induce IAP activity in GF animals. In contrast, regulation of the galactoconjugate requires colonization with constituents of the indigenous microbiota. We have further shown that these two independent pathways, using different microbial signals, regulate host responses through the transcriptional regulation of the IAP and galactosyltransferase (GaIT) genes. In this grant we will investigate the molecular basis and functional significance of these two signaling pathways between the host and its microbiota. Our preliminary data demonstrate that IAP activity, which has been shown to dephosphorylated LPS, is required in the intestine to protect against LPS toxicity. We will test the hypothesis that the normal function of this intestinal enzyme is to detoxifies LPS and reduces intestinal inflammation in response to gram-negative commensal and pathogenic bacteria. To investigate the function and microbial regulation of galactoconjugate expression in the zebrafish we have established a zebrafish mono-association model with a genetically tractable bacterium, Aeromonas veronii biovar sobria. We will use bacterial genetic and zebrafish transgenic approaches to identify the microbial signals and the host pathways that regulate GalT transcription. Finally, we will test the hypothesis that intestinal Gala1,3Gal glycan expression promotes the establishment of the appropriate microbiota. The proposed research will teach us about the molecular dialogues in which we engage with our microbiota and will shed light on the ways in which this dialogue is disrupted in diseases such as Gl inflammatory disorders, opportunistic infections, and sepsis.